Oligomers containing an olefinic end group are known in the art. For example, U.S. Pat. No. 5,362,826 discloses a process of obtaining terminally unsaturated oligomers (.omega.-ethylenically unsaturated oligomers). Such oligomers are known to be useful as non-metallic chain transfer agents.
Terminally unsaturated oligomers can be made by a number of conventional means. One way is to employ metal-containing chain transfer catalysts, for example, consisting of cobalt (II or III) chelates such as disclosed in U.S. Pat. No. 4,680,352, U.S. Pat. No. 4,694,054, U.S. Pat. No. 5,324,879, or WO 87/03605 published Jun. 18, 1987.
Conventional wisdom had been that, once formed, terminally unsaturated oligomers, including dimers and higher molecular weight species, are essentially inert to the oligomerization process. This belief is based upon the observation that the oligomers cannot be homopolymerized or copolymerized with methacrylic monomers to a reasonable extent. For example, K. J. Abbey et al., in J. Polymer Science: Part A: Polymer Chemistry, 31, p. 3417-3424 (1993) believe this is due to the instability of intermediates formed after addition of propogating radicals to the oligomers. The radicals are so sterically hindered that they cannot react with another oligomer or monomer and decompose (in a so-called .beta.-scission reaction) to yield initial oligomer or the like. This has now been confirmed in blank experiments (see Comparative Examples 3 and 5 below), which show that, without an effective amount of chain transfer catalyst, incorporation into polymer occurs to only a negligible extent.
Surprisingly, it has now been found that a metal-containing chain transfer catalyst can be used to reinitiate the terminated chains of a terminally unsaturated oligomer for further oligomerization. This has led to an improved polymerization process for making terminally unsaturated oligomers having a controlled degree of polymerization (DP) in the polymerization process without a significant yield loss or disposal problem.